It is a meteorite that's mostly iron with some nickel, chromium and other stuff. There have been many large impacts throughout history, like the one that caused Meteor Crater in Arizona. There have been big ones in Argentina, Russia, Norway, the US and all over. People search the surrounding area with metal detectors and find them. They can tell by the composition and grain structure that it's a meteorite and not an Earth bound rock.
They formed as lumps of materials, leftovers from a supernova explosion, the same way that planets formed billions of years ago. Most were sucked up into planets as they formed, but some remained as the asteroids between Mars and Jupiter. They were kept from forming a planet by Jupiter's gravity. As they cooled over millions of years, the heavier elements sank to the bottom, just as Earth has an iron core. Something in recent history, like a passing comet might have caused some of them to collide and send one towards Earth. They go so fast that upon impact with the atmosphere about 5 miles up, they usually explode. This causes a debris field that covers several miles. There are people that do nothing but hunt for those and sell them on the open market. I saw a program on Discovery where a team of guys found huge iron meteorites in Kansas fields with some green glass bubbles inside. They had a huge 20 foot wide detector trailing their truck, and when they found something, they had to dig down around 4 feet. It's a very lucrative market, and they enjoy the hunt. They get bought and sold by collectors.